U.S. Pat. No. 4,979,477 is incorporated herein by reference and discloses how the carburetor setting can be made recognizable for an operator by an optical display such as a luminescent diode or the like. The luminescent diode is switched on or off by a control unit in dependence upon the comparison of an actual signal to a desired limit value. In a first operating state, the luminescent diode is switched off and thereby shows that the actual signal lies outside of the window determined by the desired limit values, that is, that the carburetor setting is poor. In its second operating state, the luminescent diode is switched on and shows thereby that the actual signal lies within the window determined by the limit values, that is, the carburetor is optimally adjusted.
If the optical display (luminescent diode) is switched off, the operator recognizes that the carburetor setting is incorrect but does not know whether the carburetor adjusting screw (for example, the idle adjusting screw) should be rotated in or rotated out. Accordingly, it can happen that the operator leans the idle mixture because of an incorrect rotation of the adjusting screw so much that an ignitable mixture is no longer present and the engine dies.